1. Field of the Invention
The present invention relates generally to buffer circuits having hysteresis such a Schmitt trigger circuits and, in particular, to buffer circuits which can be disabled and enabled while retaining the hysteresis characteristic that existed prior to disablement.
2. Description of Related Art
Level detection circuits having hysteresis, sometimes referred to as Schmitt trigger circuits, are commonly used as input circuits of integrated circuits for receiving signals external to the integrated circuit. Referring to the drawings, FIG. 1 is a schematic diagram of an exemplary level detection circuit, generally designated by the numeral 10. The circuit includes four MOS transistors connected in series, with the common gate connection forming the input to the circuit. Transistors 12 and 14 are P-type devices, with the source of transistor 12 connected to a voltage supply V.sub.DD, typically +5 volts, and the source of transistor 14 connected to the drain of transistor 12.
The common drain connection of P-type transistor 14 and N-type transistor 16 functions as the circuit output. The source of transistor 16 is connected to the drain of N-type transistor 18, with the source of transistor 18 being connected to the circuit common (V.sub.SS). The circuit output is connected to the common gates of a P-type transistor 20 and an N-type transistor 22. Transistor 20 has a drain connected to the circuit common and a source connected to the junction of transistors 12 and 14. Transistor 22 has a drain connected to supply V.sub.DD and a source connected to the junction of transistors 16 and 18.
FIG. 2 is a diagram showing an idealized transfer function characteristic curve 24 of the FIG. 1 circuit. The input threshold voltage of circuit 10, the point at which the input voltage Vin is equal to the output Vout voltage, is a function of the state of the output as can be seen from curve 24. The circuit threshold voltage is to be distinguished from the threshold voltage of the individual transistors, that being defined as the gate-source voltage necessary to cause the transistor to conduct some minimum current, such as 1 .mu.A, for a particular drain-source voltage, such as 1 volt.
Assume, for example, that the circuit input Vin is at a low level near ground (V.sub.SS) and is increased to a high level. Also assume that the output Vout is at a high level approaching V.sub.DD. At this point, N-type transistors 16 and 18 will be off and N-type transistor 22 will be on, but will not be conducting current. Further, the P-type transistors 12 and 14 will be on and P-type transistor 20 will be off.
As the input Vin increases in voltage, the gate-source voltage of transistor 18 will increase until the threshold voltage of the transistor V.sub.TN18 is reached at which point current from transistor 22 will start to flow through transistor 18. The source voltage of transistor 16 will thus increase due to transistor 22 thereby tending to increase the effective turn on point of transistor 16. Arrow 26A of FIG. 2 indicates that portion of the transfer function curve 24 that pertains to these conditions once Vin has reached a first threshold voltage Vin of the circuit.
Eventually, the input voltage Vin will be equal to the drain-source voltage of transistor 18 plus the gate-source threshold voltage of transistor 16 (V.sub.TN16) so that transistor 16 will proceed to turn on. This point is the second threshold voltage Vin.sup.+ of the circuit. At the same time, transistors 12 and 14 will start to turn off. Transistor 22 will also begin to turn off and transistor 20 will begin to turn on. Transistors 20 and 22 provide positive feedback which will increase the rate at which the circuit 10 changes state. As indicated by arrow 26B, the output Vout will then drop to ground potential (V.sub.SS).
When output Vout is low, transistor 20 will be on but will not be conducting current because transistors 12 and 14 will be off. Should Vin begin to drop, the voltage will eventually reach the second threshold voltage Vin.sup.+. However, since transistor 22 is not conducting, the overall threshold voltage of the circuit will be lower so that transistors 16 and 18 will continue to remain conductive. As Vin is reduced further, Vin will be equal to the sum of the drain-source voltage dropped across transistor 20 plus the gate-source threshold voltage V.sub.TP14 of transistor 14. Vin will then be at the first threshold voltage Vin.sup.- and transistor 14 will proceed to turn on together with transistor 12. As indicted by arrow 26D of FIG. 2, the output voltage Vout will switch back to the high state (V.sub.DD). One of the properties of the Schmitt trigger circuit 10 is increased immunity to noise on the input Vin. By way of example, when input Vin has increased just to the point Vin.sup.+ so that the output has changed to a low state (V.sub.DD), noise on the input may cause Vin to momentarily drop back below Vin.sup.+. However, due to the hysteresis effect provided by transistor 20, the threshold voltage of the circuit will have dropped to Vin.sup.- so that there will be no change in output Vout unless the noise is sufficiently great to cause the input to drop even further to Vin.sup.-.
In many applications, it is necessary to switch an integrated circuit to a low power or standby mode of operation to conserve power. This technique is sometimes used when several integrated circuits are connected to receive a group of input signals from an external source, with only one of the circuits being active at any time and with the remaining circuits being in a standby mode. Input buffer circuits of such integrated circuits must be powered down in this mode of operation to prevent the circuits from drawing DC and AC switching currents, particularly due to changes in the external inputs. The most common method of powering down such interface circuits is to disconnect the circuit from the power source. If this approach were used for the level detection circuit 10, the circuit will come up in an indeterminate state when power is reapplied. Thus, the input threshold voltage could be either Vin.sup.- or Vin.sup.+ independent of the state of the circuit when power was removed. This can result in improper operation when the circuit is first accessed after power has been reapplied.
The present invention overcomes the above-described limitations of conventional level detection circuits having hysteresis. Reliable level detection with hysteresis is provided by the circuit even after the circuit has been placed in a low power mode of operation. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.